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Rejection rationale

This FTP is rejected

The problem of transmitting large messages between peers over FIDL is important, a very current concern faced by developers, and a current deficiency of FIDL. The combination of channel limits, the ease of defining messages which can surpass those limits depending on runtime behavior, along with the inherent difficulty to size messages (e.g. max out pagination) make this problem untractable in application code (at least, at scale). A solution must be provided by FIDL.

In short, FIDL needs to provide a way for library authors to eliminate the possibility of runtime surprises due to protocol limits (e.g. hard limits of number of bytes and handles in the case of Zircon channels), and FIDL developers need to be able to rely on bindings to implement protocols which may exchange large messages.

This FTP has many desirable ingredients, and will serve as a reference in the future. In particular:

  • The problem statement and motivation.
  • Favoring static verification to provide guarantees that either messages satisfy the protocol limits, or opting into a runtime mechanism offered by the bindings to 'make it fit' (possibly incurring some extra cost)
  • Relying on a distinction between value types vs resource types to offer different points in the convenience/performance tradeoff continuum.

Despite the above, this FTP is rejected for the following reasons:

  • How a messages' bytes and handles are transmitted from one peer to another is not a wire format concern, and there should be no modification to the wire format needed to satisfy the requirements.
  • The introduction of the box<T> notation helps identify a place in a message where boxing can occur, i.e. it is a fine grained mechanism to indicate boxing behavior at an (almost) arbitrary place in a message. Instead, the current thinking is that providing an annotation at the method level is preferable, i.e. a coarser description mechanism, at use site rather than declaration site.
  • With the gradual generalization of FIDL to support different transports beyond Zircon channels, we must take into consideration that each transport will come with its own limitations. For instance, as we look to expand FIDL to Zircon fifos, the size limitation will be different, and no handles can be allowed. This furthers the view that a method level annotation (and not a type centric annotation) is preferable.

Today, an approach taken to address this in applications uses the fuchsia.mem/Data type, a union representing data either inline or in a VMO. Generalizing this approach to any request and/or response using syntactic sugar as is done for errors, along with bindings support, is the current top contender to move forward on this topic.

FTP-017: Box<Knox>

Field Value
Status Rejected
Submitted 2018-09-27
Reviewed 2020-06-17


Introduce a mechanism for transmitting large structured data objects through an auxiliary VMO associated with a FIDL message.


There is an upper bound on the maximum size of FIDL message that can be transmitted between processes through a Zircon channel, as defined by ZX_CHANNEL_MAX_MSG_BYTES. At the time of this writing, the limit is 64 KB including FIDL headers.

Although the limit is adequate for many applications, it is occasionally necessary to transmit larger objects. This poses a challenge for FIDL API designers since they must design alternative means for transmitting those objects, such as:

  • Pagination: When transmitting collections of objects (typically vectors), deliver objects incrementally in batches rather than all at once.
    • Works well as long as each individual object is smaller than the limit (taking headers and other fields into account).
    • Somewhat difficult for developers to determine how to pack objects efficiently into messages without exceeding the limit due since there are currently no APIs for estimating the size of a FIDL message or for incrementally constructing one.
  • VMO Encapsulation: Instead of transmitting large objects within the FIDL message itself, copy them into a VMO. Typically this is represented as a fuchsia.mem/Buffer.
    • Works well for byte vectors (blobs).
    • Cumbersome for structured data objects since the developer is responsible for invoking serialization/deserialization.
    • Requires discipline to mitigate security threats due to shared memory.

Failure to anticipate this issue is a major cause of runtime instability.

We believe that FIDL should offer a built-in, safe, statically verifiable, and efficient mechanism for transmitting large data objects.


Boxes in Brief

A box is a container for a possibly large data object which may need to be transmitted out of band when the total message size (including headers) exceeds the limit for a Zircon channel[1].

A box only holds data objects; it cannot hold objects that have handles[2].

At design time, a FIDL protocol author..

  • encapsulates data objects into boxes when they anticipate that messages containing those objects could exceed channel limits

At compile time, the FIDL compiler...

  • parses declarations
  • statically verifies that each box only contains data objects (no handles)
  • statically verifies that the maximum possible size of each FIDL message does not exceed channel limits (in static message size enforcement mode)
    • assumes vectors and strings are as large as their bounds will allow
    • assumes that extensible unions and structs (tables) are as large as they could possibly become
    • rejects recursive structures unless the recursion is broken by a box

At compile time, each FIDL code generator...

  • produces code sufficient for serializing and deserializing boxed data

At runtime, the FIDL encoder..

  • packs as many boxed objects as will fit into the message body, following all other out-of-line objects
  • packs all remaining boxed objects that didn’t fit into a VMO

At runtime, the FIDL decoder..

  • prior to accessing the VMO containing boxed objects (if any), ensures that it has the only handle to the VMO (the VMO is not shared)
  • extracts boxed objects from the message body and from the VMO

Language Details

We introduce a new built-in FIDL type, denoted box<T>. T designates the type of object to be put in a box.

  • T must be a reference type that does not directly or indirectly contain handles.
  • T must not be a primitive type.
  • T may be an optional type.

The new type box<T> can be used wherever reference types such as structs, unions, vectors, and strings would be accepted.

Example Box Types

  • box<string>
    • a box containing an unbounded string
  • box<int>
    • ERROR: a box cannot contain an object of primitive type
  • box<vector<T>:100>
    • a box containing a bounded vector of T objects
  • box<vector<T>>
    • a box containing an unbounded vector of T objects
  • vector<box<T>>:100
    • a bounded vector of boxed T objects.
  • box<string:100>
    • a box containing a bounded string
  • box<string>
    • a box containing an unbounded string
  • box<MyStruct>
    • a box containing a structure
  • box<MyStruct?>
    • a box containing an optional structure
  • box<MyStruct>?
    • ERROR: a box cannot be optional

Example Declarations

interface Database {
    // OK
    1: SelectTop(string:1000 query) -> (box<Record> record);

    // ERROR: reply may exceed message size limit
    // consider wrapping large objects in a box<>,
    // “Record” size is unbounded
    2: BadSelectTop(string:1000 query) -> (Record record);

    // OK
    3: SelectAll(string:1000 query) -> (box<vector<Record>> records);

    // ERROR: reply may exceed message size limit
    // consider wrapping large objects in a box<>,
    // “vector<Record>” size is unbounded
    4: BadSelectAll(string:1000 query) -> (vector<Record> records);

struct Record {
    string name;
    string address;

Wire Format

(Section is incomplete.)

Idea 1: during depth-first traversal for serialization add all boxes encountered to a queue, once finished first pass pack boxed items in order following out-of-line objects until no more space remains then calculate size of remaining boxed objects, allocate a single VMO, and continue packing box contents from there

Idea 2: like Idea 1 but put each box into its own VMO, slightly simpler to implement but may be more limiting

Idea 3: maybe we should drop the idea of boxes altogether and do something at the level of a method instead, like an annotation, e.g. [Huge]


(Section is incomplete.)

Enhanced VMO Syscalls

(Section is incomplete.)

Idea 1: define “ensure not shared” flag, verify VMO has exactly one handle, shares no pages with other VMOs, and is not mapped, can pass this flag to zx_vmo_read/write/map, etc.

Idea 2: define a new syscall to check if VMO is unshared

Idea 3: make it really check to reverse-COW snapshot a VMO if it is already unshared (should be a no-op)

Idea 4: punt on VMOs and use Views instead

Implementation Strategy

(Section is incomplete.)


(Section is incomplete.)

Documentation and Examples

(Section is incomplete.)

Backwards Compatibility

In addition to providing a mechanism for transferring large data objects, boxes are also intended to address static safety concerns.

Currently, programs that attempt to transmit FIDL messages that exceed channel limits will fail at runtime leading to system instability. Once boxes are available, it should be feasible to introduce static message size enforcement in the FIDL compiler such that we can guarantee at compile time that no message will ever exceed channel limits; the excedent content can simply be moved into a box by the FIDL protocol author.

However, enabling static message size enforcement all at once may break existing code and hinder migration efforts.

We propose to resolve this problem as follows:

  • Initially, enable static message size checks in permissive mode.
    • The FIDL compiler should check message sizes and emit warnings when channel limits could possibly be exceeded. Recommend that the FIDL protocol author start using boxes instead.
  • Proceed with migration.
  • When complete, enable static message size checks in enforcement mode.
    • The FIDL compiler should check message sizes and emit errors when channel limits could possibly be exceeded. Halt compilation.


(Section is incomplete.)


By replacing existing ad-hoc mechanisms with an official solution supported by FIDL language bindings, we have an opportunity to improve overall security discipline.

For example, the FIDL language bindings can ensure that a VMO containing boxed data only has a single owner before attempting to access its content. This resolves common shared memory threats such as:

  • The provider of a VMO modifies data while it is being accessed by the client.
  • The provider of a VMO changes the size of the VMO while it is being accessed by the client or otherwise induces the client to page fault.

Conversely, introducing this feature may contribute to increased use of large messages and therefore increase the likelihood of other threats such as:

  • The provider of a VMO sends an enormous reply to a client, causing the client to allocate a lot of heap while deserializing it.


(Section is incomplete.)

Drawbacks, Alternatives, and Unknowns

(Section is incomplete.)

Prior Art and References

(Section is incomplete.)

Editor's note: Rejected FTP-005: Method Impossible which proposed banning all methods which could go beyond protocol limits. This is too constraining due to the limitation of static analysis which prevent capturing runtime behavior working to properly paginate messages, or manually 'box' them.


Hypothetically, FIDL could be transmitted over other channels for which boxing may take on a different nature. How that would be implemented is out of scope for this proposal.


Editor's note: Since this FTP was authored in 2018, the distinction between value types and resources is formally part of the FIDL language, see FTP-057.